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GSHP temperature range

We have a ground-source heat pump project where the temperatures range from 40F-90F. We have ~20 ton AC units with enconomizer coils on the returns to switch over when the temperature gets low - but we aren't sure how low to go. How difficult would it be to change the temperature setting for the enconomizer on the fly? Should we test run them to find a set point to prevent freezing and still provide adequate cooling?

I've only worked with a few GSHP's, but the heat transfer theory is the same, only different mediums of transfer. Assuming your loop has glycol and assuming you are talking about freezing up the indoor coil it will be dependent upon what type of refrigerant you are talking about. R-22 below around 200 PSIG High side will likely start to potentially cause freeze up issues on the indoor coil. Now if you have a 'wet economizer' coil in your returns that your are simply going to pump the ground loop solution through a coil work it similarly as you would work a regular economizer and then you don't have to worry about freezing your indoor DX coil. Once your GS loop temp gets below say 55 deg, (make this adjustable), lockout DX cooling and allow your 'wetside economizer' to provide the only means of cooling.

Does this system have a controls contractor working with you guys or are you doing it? Also who designed this project? Is this an engineer stamped project? Honestly is sounds like you should get the GSHP manufacturer/engineer involved in this so they can give you some more insight.

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